1. Field of the invention
The present invention relates in general to a motor vehicle power transmission in which all rotating members are at rest when a motor vehicle is stopped, and more particularly to a lubricating device for such a vehicle power transmission, which uses a mechanical oil pump driven by a rotating member of the power transmission.
2. Discussion of the Related Art
A motor vehicle power transmission system including a speed changing or reducing device and a differential gear device is equipped with a lubricating device for lubricating various rotating members disposed therein, so as to protect bearing portions and mutually meshing portions of the rotating members from seizure and early wearing. There have been proposed various types of such a lubricating device for the vehicle power transmission system. One example of the known lubricating device is a so-called "oil bath lubricating device", wherein bearings and other portions of the power transmission system to be lubricated (hereinafter referred to as "lubricating points" where appropriate) are immersed in a bath of a lubricating oil and are thus lubricated by the oil during operation of the power transmission. Another example of the known lubricating device is a so-called "forced-feed lubricating device", wherein the lubricating oil is pressurized, and the pressurized oil is fed to the lubricating points. Two types of the forced-feed lubricating device are known, namely, a mechanically operated forced-feed lubricating device and an electrically operated forced-feed lubricating device. In the mechanically operated forced-feed lubricating device, an oil pump for delivering the pressurized lubricating oil is driven by an appropriate rotating member of the power transmission system. In the electrically operated forced-feed lubricating device, an electrically operated pump is employed to deliver the pressurized lubricating oil.
In the oil-bath lubricating device as described above, the lubricating oil is agitated or stirred and scattered by the rotating members of the power transmission system when these rotating members are rotated. Accordingly, the motor vehicle tends to undesirably suffer from a relatively large energy loss when the vehicle is running at a comparatively high speed. When this oil-bath lubricating device is used in an electric motor vehicle, for example, the travel distance or mileage of the vehicle per one battery charging is undesirably reduced. Further, the lubricating oil is likely to be overheated, leading to bubbling and early deterioration of the oil, which may cause easy leakage of the oil.
Unlike the oil-bath lubricating device, the mechanical forced-feed lubricating device is not likely to suffer from a large energy loss. In the electric motor vehicle, however, all of the rotating members are at rest when the vehicle is stopped. Accordingly, a mechanical oil pump which is driven by a rotating member of the power transmission is turned off upon stopping of the electric motor vehicle, and therefore the lubricating oil is no more supplied to the lubricating points in the transmission system. This means that the power transmission system cannot be adequately lubricated upon restarting of the vehicle after a stop. Described in detail, upon starting of the vehicle, the power transmission system starts to be rotated and accordingly, the oil is delivered by the mechanical pump and gradually supplied to the lubricating points. However, it takes some time before the oil reaches the lubricating points. Upon and immediately after starting of the vehicle, the rotating speed of the power transmission system is relatively low, and accordingly the amount of the lubricating oil delivered from the pump is small. However, a large amount of the lubricating oil is required to sufficiently lubricate the transmission system upon and immediately after starting of the vehicle, since a comparatively large torque is transmitted to the power transmission system, and an accordingly large amount of load acts on the bearing and meshing portions of the transmission system. When the vehicle wheels slip on the road surface upon starting of the vehicle, the gears of the differential gear device are rotated at a high speed with a large amount of load acting thereon, and therefore a large amount of lubricating oil is required to lubricate the gears.
In the electrically operated forced-feed lubricating device, an electrically operated oil pump is capable of delivering the lubricating oil to the lubricating points, irrespective of the rotating speed of the power transmission system. Thus, the electrically operated forced-feed lubricating device is preferably employed for an electric motor vehicle wherein all rotating members of the power transmission system are turned off upon stopping of the vehicle. However, this lubricating device which requires an electric motor for driving the electric oil pump suffers from a shorter service life, and requires more frequent inspection and replacement of the components, than the mechanically operated forced-feed lubricating device. For the electrically operated forced-feed lubricating device to provide a sufficient lubricating effect upon starting of the vehicle, the electric pump motor should be kept in the operating state so as to enable the lubricating oil to be continuously delivered from the oil pump to the lubricating points even while the vehicle is at a stop, since an operation of an accelerator pedal by the vehicle operator to restart the vehicle is unexpected. In this case, the electric power consumed by the electric pump motor for continuous operation of the oil pump is generally larger than the energy loss experienced in the mechanically operated forced-feed lubricating device wherein the mechanical pump is driven by a rotating member of the power transmission system.
Japanese Patent Application No. 6-190253 (filed Aug. 12, 1994), which has not been laid open at the time the present invention was made, discloses a power transmission system including a lubricating device, wherein oil-bath lubrication is effected during running of the vehicle at a relatively low speed, while forced-feed lubrication using a mechanical oil pump is effected during running of the vehicle at a relatively high speed. Further, an excess of the lubricating oil delivered from the mechanical pump during running of the vehicle at a high speed is stored in an oil reservoir, so as to reduce the level of the oil in the housing of the transmission system for thereby preventing an energy loss due to agitation of the oil by the rotating members rotating at a high speed. An example of this type of power transmission system for an electric motor vehicle is generally indicated at 100 in FIG. 9. This power transmission system 100 has an axis of rotation or centerline O, and includes a speed reducing device 102 of planetary gear type and a differential gear device 104 of bevel gear type, which are disposed concentrically or coaxially with the centerline O. A rotary motion of an output shaft 106 of an electric motor, which is shown in a left-hand side part of FIG. 9, is transmitted to a left and a right drive wheel of the electric motor vehicle via the speed reducing device 102, differential gear device 104 and a pair of output members 108, 110 of the power transmission 100. Thus, the motor shaft 106 serves as an input shaft of the transmission system 100. The speed of the motor shaft 106 is reduced by the speed reducing device 102 before the motion is transmitted to the differential gear device 104. The speed reducing device 102 has a carrier 112 which includes a differential casing portion 112a that constitutes a part of the casing of the differential gear device 104. A pump drive gear 114 is fixed to the outer circumferential surface of the differential casing portion 112a, so that a mechanical oil pump 118 in the form of a trochoid pump is driven by the drive gear 114 through a driven gear 116.
The lubricating oil sucked up from an oil sump 120 and pressurized by the mechanical pump 118 is supplied into the casing of the differential gear device 104 (partially constituted by the differential casing portion 112a), via an oil passage 124 formed in a housing 122 of the power transmission system, an annular oil passage 128 formed in a differential cover 126, and radial and axial oil passages 130, 132, that are formed in the output member 110, so that the bearing and meshing portions of the gears of the differential gear device 104 are lubricated by the lubricating oil delivered from the mechanical oil pump 118. A portion of the oil supplied to the axial oil passage 132 is further fed through an oil passage 134 formed in the output member 108, and an annular oil passage 136 formed between the output member 108 and the motor shaft 106, so that the bearings and other portions of the motor shaft 106 and output member 108 are lubricated. The oil is further fed to the electric motor for cooling this motor, and is returned back to the oil sump 120. A portion of the oil supplied to the oil passage 128 is further fed to three pinion shafts 142 fixed to the carrier 112, through oil passages 138 formed in the differential cover 126, and oil passages 140 formed in the differential casing portion 112a of the carrier 112, so that the bearing portions of three stepped composite pinion gears 144 mounted on the respective pinion shafts 142 are lubricated. In the present lubricating system, three sets of oil passages 138 and 140 are provided for the three pinion shafts 142, respectively.
The housing 122 has an oil reservoir 146 for storing an excess of the lubricating oil supplied to the oil passage 124. The amount of the lubricating oil in the housing 122 is determined such that the level of the oil when the vehicle is at rest is in the neighborhood of the centerline O, so that the rotating members of the power transmission system 100 are lubricated primarily by oil-bath lubrication immediately after starting of the vehicle and during running of the vehicle at a relatively low speed. During running of the vehicle at a medium or high speed, a sufficient amount of the lubricating oil is delivered from the mechanical pump 118 to the lubricating points such as the bearing portions of the gears of the differential gear device 104 and the bearing portions of the composite pinion gears 144 of the speed reducing device 102. When the amount of delivery of the mechanical pump 118 exceeds a given upper limit due to a rise of the vehicle running speed, the lubricating oil begins to be fed into the oil reservoir 146, and the level of the oil bath within the housing 122 is accordingly lowered, eventually to a level lower than the lower ends of the speed reducing device 102 and differential gear device 104. Thus, the transmission system 100 is sufficiently lubricated by oil-bath lubrication during starting and low-speed running of the vehicle, and at the same time an otherwise possible energy loss due to agitation of the oil during high-speed running of the vehicle is effectively prevented. The oil stored in the oil reservoir 146 slowly drops by gravity down to the lubricating points, and can be utilized to lubricate the transmission system 100 immediately after starting of the vehicle following a comparatively short stop.
In the lubricating device as disclosed in FIG. 9, however, it takes a considerable time until a large amount of the lubricating oil is stored in the oil reservoir, particularly where the vehicle running speed is raised from a low level to a high level at a comparatively high rate. In this case, it requires a long time until the level of the oil in the housing is lowered enough to avoid an energy loss due to agitation of the oil by the rotating members of the transmission system. Accordingly, this lubricating device inevitably suffers from some amount of energy loss. Further, the provision of the oil reservoir results in an increase in the cost of the lubricating device, and an increase in the required size of the housing including the reservoir, whereby the ease of installation of the transmission system on the vehicle is deteriorated. In this respect, it is noted that a large amount of lubricating oil is required to permit sufficient lubrication of the transmission system during starting of the vehicle, and that the oil reservoir should have a sufficiently large oil storage capacity to assure elimination of the energy loss due to agitation of the oil during high-speed running of the vehicle.